1. Field of the Invention
The present invention is related to the field of telephony, and more specifically to devices, software and methods for encoding while compressing DTMF tones.
2. Description of the Related Art
A feature that is increasingly used in telephony is passing Dual Tone Multi Frequency (DTMF) DTMF tones over a regular telephone connection. These tones are emitted by the user pushing the buttons on his telephone, while a telephone line is held open. These tones are passed through the telephone line along with voice, and are recognized by the remote system at the other end of the telephone line.
The DTMF tones are passed for a variety of purposes. One such purpose is to give answers to an automated remote system, for better routing of a call. Such systems are said to employ interactive voice response (IVR), and are used in increasing numbers of applications, for example in telephone banking. IVR permits automating a remote system, and serving many customers at once, with fewer delays.
An evolving challenge is to transmit regular telephone conversations while using compression. That may occur if the transmission is wireless. It may also occur if the transmission is over a network such as the internet, as opposed to over a regular, conventional telephone line. In that case, the voice of the conversations will be sent in packets under a voice over internet protocol (VoIP).
A problem arises when compressing DTMF sounds. That is because the DTMF sounds emitted from the user""s telephone might not be recognized at the other end. The reason is as follows.
DTMF signals consist of two tones, a high frequency tone H and a low frequency tone L. Each of these tones has an amplitude Am(tone). The difference in amplitudes is called the twist T, which is preferably measured in decibels (dB). In particular,
xe2x80x83T=Am(H)xe2x88x92Am(L)xe2x80x83xe2x80x83Equation (1)
By their nature, conventional telephone lines attenuate (in other words, diminish the amplitude) of the high frequency tone H. If this problem were left unattended, then according to Equation (1), there would be less twist T at the other end. So much less, that it might not enable IVR over conventional telephone lines. This problem has been addressed by some telephones preemphasizing the amplitudes of the high frequency. In other words, Am(H) is increased. This will permit IVR over regular telephone lines.
A problem arises due to the preemphasis and other reasons associated with compression. After the encoded twist is decoded, it may be outside a recognition range of the decoder. In that case, the DTMF sound is not recognized, and the IVR simply does not work. This should not be surprising, because encoders and decoders (codecs) were initially designed for passing voice signals, not DTMF tones.
Nevertheless, it is desirable to be able to pass DTMF tones through telephone connections, even when low bit rate voice coding is used, to implement IVR and other features. It is so desired whether the telephone device of the user preemphasizes amplitude of the high frequency or not.
The present invention overcomes these problems and limitations of the prior art.
Generally, the present invention provides devices, software and methods for encoding DTMF signals for telephone connections between digital voice communication systems that use compression, such as wireless connections or network connections. The invention teaches to determine the twist of the DTMF tone in the signal, and to determine whether the twist is within a mapped range of a decoder. If not, the signal is first balanced to change the twist, and then the balanced signal is encoded.
As a result, when the encoded signal is eventually decoded, the resulting DTMF tone will have a twist that will be within the range of remote decoder. This way the DTMF tones will be perceptible for IVR.
The invention will become more readily apparent from the following Detailed Description, which proceeds with reference to the drawings, in which: